1. Field of the Invention
The present invention relates to a valve operating system for a multicylinder internal combustion engine having a single overhead camshaft above the cylinders with three intake cams engaging three intake rocker arms that are selectively coupled and uncoupled for operating two intake valves and two exhaust cams engaging two exhaust rocker arms that operate two exhaust valves for each cylinder.
2. Description of the Prior Art
A valve operating system for an internal combustion engine is already known, for example, from Japanese Patent Application Laid-open No. 57806/88, which comprises first, second and third intake-side cams and first and second exhaust-side cams for each cylinder all provided on a single overhead cam shaft, first and second intake-side rocker arms operatively connected independently to a pair of intake valves for each cylinder and a third intake-side rocker arm disposed between the first and second intake-side rocker arms, which arms are swingably carried on an intake-side rocker shaft in engagement with the first, second and third intake-side cams, respectively, first and second exhaust-side rocker arms operatively connected independently to a pair of exhaust valves for each cylinder and swingably carried on an exhaust-side rocker shaft in engagement with the first and second exhaust-side cams, respectively, and a connection switchover mechanism provided on the intake-side rocker arms capable of switching-over the connection and disconnection of the adjacent intake-side rocker arms.
In the above-described prior art valve operating system, however, the intake-side rocker arms are disposed adjacent one another in a section where the connection switchover mechanism is provided, but cams for the intake-side rocker arms and cams for the exhaust-side rocker arms are provided on the cam shaft adjacent one another in an axial direction. Therefore, the intake-side rocker arms cannot be disposed in a compact arrangement, resulting in an increase in size of even the connection switchover mechanism and an increase in weight of the intake-side rocker arms, and resulting in difficulty maintaining the dimensional accuracy of the connection switchover mechanism.
As a result, the present applicant has already proposed a valve operating system (in Japanese Patent Application Laid-open No. 1405/92) in which the exhaust-side rocker arms are disposed on opposite sides of the three intake-side rocker arms in a position corresponding to the cam shaft, thereby providing a compact arrangement of the valve operating system. In this proposed technique, two or all of the three intake-side rocker arms are in direct sliding contact with the cams on the cam shaft whereby there is a significant friction loss in the valve operating system. In order to reduce the loss by friction, each of the intake-side rocker arms may be put into contact with the cam through a roller for a low-friction rolling contact. However, if the roller is supported by a pin on the intake-side rocker arm, the width of the intake-side rocker arm along the axis of the cam shaft in such roller area is relatively large, and the space for the rocker arms is large in a direction along the axis of the cam shaft. This is not desirable from the viewpoint of a compact arrangement of the engine.
In addition, from the view point of an improvement in combustibility of the engine, it is desirable that the angle formed by axes of the intake valves and the exhaust valves projected on a plane including an axis of the cylinder is small. However, if the interference of the intake-side rocker arms and the exhaust-side rocker arms is avoided to provide a reduction in such angle, the space for the rocker arms may be increased in the direction along the axis of the cam shaft, again resulting in difficulty in disposing the rocker arms in a compact arrangement.